A fluid in the supercritical state is referred to as a supercritical fluid. A fluid enters the supercritical state when it is subjected to a combination of pressure and temperature at which the density of the fluid approaches that of a liquid. Supercritical fluids exhibit properties of both a liquid and a gas. For example, supercritical fluids are characterized by high solvating and solubilizing properties that are typically associated with compositions in the liquid state. Supercritical fluids also have a low viscosity that is characteristic of compositions in the gaseous state. Supercritical fluids have been adopted into common practices in various fields. The types of applications include pharmaceutical applications, cleaning and drying of various materials, food chemical extractions, and chromatography.
Supercritical fluids have been used to remove residue from surfaces or extract contaminants from various materials. For example, as described in U.S. Pat. No. 6,367,491 to Marshall, et al., entitled “Apparatus for Contaminant Removal Using Natural Convection Flow and Changes in Solubility Concentration by Temperature,” issued Apr. 9, 2002, supercritical and near-supercritical fluids have been used as solvents to clean contaminants from articles; citing, NASA Tech Brief MFS-29611 (December 1990), describing the use of supercritical carbon dioxide as an alternative for hydrocarbon solvents conventionally used for washing organic and inorganic contaminants from the surfaces of metal parts.
Supercritical fluids have been employed in the cleaning of semiconductor wafers. For example, an approach to using supercritical carbon dioxide to remove exposed organic photoresist film is disclosed in U.S. Pat. No. 4,944,837 to Nishikawa, et al., entitled “Method of Processing an Article in a Supercritical Atmosphere,” issued Jul. 31, 1990. Particulate surface contamination is a serious problem that affects yield in the semiconductor industry. When cleaning wafers, it is important that particles and other contaminants such as photoresist, photoresist residue, and residual etching reactants and byproducts be minimized.
While “high grades” of CO2 are available commercially, calculations show that given the purity levels of delivered CO2 it is all but impossible to avoid particle formation on a substrate during supercritical carbon dioxide processing.
There is a need for removing contaminants and particles from a fluid such as carbon dioxide.